Semiconductor devices such as a semiconductor integrated circuit device and others are mass-produced by repeating a photolithography process of irradiating a mask, which is an original master on which a circuit pattern is drawn, with exposure light to transfer the circuit pattern onto a semiconductor wafer (hereinafter, simply referred to as a wafer) via a reduction optical system.
Recently, miniaturization of the semiconductor devices has been advanced, and a method of improving the resolution by further reducing the exposure wavelength of photolithography has been studied. More specifically, although ArF lithography using argon fluoride (ArF) excimer laser light with a wavelength of 193 nm as exposure light has been developed up until now, EUV lithography using EUV light with a wavelength of 13.5 nm which is far shorter than that described above is under development. Note that the EUV light is also referred to as a soft X-ray.
In the EUV lithography, a transmission-type mask cannot be used due to the relevance of light absorption of substances. Therefore, a multilayer reflective substrate utilizing reflection (Bragg reflection) by a multilayer film obtained by stacking, for example, an Mo (molybdenum) layer and an Si (silicon) layer is used as a mask blank of the EUV lithography. This multilayer film reflection is the reflection utilizing a kind of interference.
A reflection-type mask for EUV lithography is constituted of a multilayer film blank obtained by depositing the above-described multilayer film on a substrate made of quartz glass or low thermal expansion glass and a circuit pattern made of an adsorption layer formed on the multilayer film blank. This reflection-type mask is a mask utilizing Bragg reflection and the wavelength of exposure light thereof is as extremely short as 13.5 nm. Therefore, even when slight variation of about one severalth of the wavelength is caused in the film thickness of the multilayer film, a local difference of the reflectance is caused, and defects called phase defects are generated at the time of transfer. Therefore, the reflection-type mask for EUV lithography has a large difference in terms of quality regarding the transfer of defects when compared with a conventional transmission-type mask.
The wavelength range of the EUV light is considered to be 9 nm to 15 nm. However, since the reflectance of the reflection-type mask and the reflective lens optical system has to be ensured when the EUV light is applied to the lithography use, the above-described wavelength of 13.5 nm is mainly used. However, the wavelength is not limited to this wavelength. For example, the wavelengths of 9.5 nm and others have been studied, and the EUV light can be applied to the lithography use as long as the wavelength thereof is within the above-described range (9 nm to 15 nm).
Moreover, in the EUV lithography, even in the case where contamination with a minute film thickness of several nm adheres to the surface of the mask, the exposure light reflectance at that part is reduced, and the so-called contamination defects which cause resolution failure, insufficient transfer accuracy, exposure in-plane dimensional variation and others become problematic.
Examples of defects of the reflection-type mask for EUV lithography are shown in FIGS. 1A to 1C. The reference numeral 201 in the drawings denotes a substrate of the reflection-type mask, 202 denotes a reflective layer made of a multilayer film, 203 denotes an absorption layer, 204 denotes an opening pattern of the absorption layer, 205 denotes a black defect remainder, 210 denotes a particle, 211 denotes a phase defect, and 220 denotes contamination. Herein, FIG. 1A shows an example of a normal black defect, FIG. 1B shows an example of a phase defect, and FIG. 1C shows an example of a contamination defect, respectively.
In the above-described phase defect and contamination defect, the reflectance of the mask reflective surface is lowered, in other words, the exposure amount is decreased, and these defects belong to black defects as a category. More specifically, in the case where the phase defect 211 is present in the opening pattern 204 formed in the absorption layer 203 as shown in FIG. 2A, when the image transferred to a photoresist film 231 on a semiconductor wafer 230 is observed, the transferred pattern 233 of the defective part has a smaller opening compared with a normal transferred pattern 232 having no defect or the opening is crashed as shown in FIG. 2B. Moreover, as shown in FIG. 3 (cross sectional view of the line A-A of FIG. 2B), in the transferred pattern 233 of the defective part, the photoresist film 231 is not removed up to the bottom.
Conventionally, as a defect correction method for the case where a black defect remainder is generated inside an opening pattern, a method of radiating FIB (focused ion beam), EB (electron beam) or the like and a method of scraping off the defect by mechanical means using a needle or the like have been used. Moreover, as a defect correction method for the case where a phase defect or a contamination defect is generated inside an opening pattern, a method in which the absorption layer 203 in the periphery of the opening pattern 204 is removed by irradiation of FIB or EB or mechanical means using a needle to enlarge the area of the opening pattern 204 as shown in FIG. 4, thereby compensating the reduction of the exposure amount has been used.
Note that a defect correction technique of the reflection-type mask for EUV lithography is described in Japanese Unexamined Patent Application Publication (Kohyo) No. 2002-532738 (Patent Document 1).